The present invention relates to a technique of correcting unevenness of brightness of a display panel such as a liquid crystal panel with high accuracy.
Display panels for performing a display using electro-optical variation of an electro-optical material, such as display panels using liquid crystal, can be classified into several kinds depending upon driving types thereof. However, active matrix display panels for driving pixel electrodes with three-terminal switching elements have approximately the following structure. That is, in this kind of liquid crystal panel, liquid crystal is interposed between a pair of substrates, and one substrate is provided with a plurality of scanning lines and a plurality of data lines to intersect each other and with pairs of a three-terminal switching element and a pixel electrode to correspond to the respective intersections. The other substrate is provided with a transparent counter electrode (common electrode) to be opposite to the pixel electrodes, and the counter electrode is kept at a constant potential. In addition, the respective opposite surfaces of both substrates are provided with an alignment film having been subjected to a rubbing process such that a major-axis direction of liquid crystal molecules is continuously twisted, for example, by about 90° between both substrates, while the respective rear-surface sides of both substrates are provided with a polarizer corresponding to the alignment direction.
Here, the switching elements provided at the intersections between the scanning lines and the data lines are turned on when scanning signals applied to the scanning lines reach an active level, and thus image signals sampled into the data lines are supplied to the pixel electrodes. For this reason, a voltage as a difference between the potential of the counter electrode and the potential of the image signals is applied to the liquid crystal layer interposed between both electrodes of the pixel electrode and the counter electrode. Thereafter, even when the switching elements are turned off, the applied voltage is kept by the liquid crystal layer itself or storage capacitors provided additionally.
At this time, light transmitted between the pixel electrodes and the counter electrode is optically rotated by about 90° depending upon degrees of twist of liquid crystal molecules when an effective voltage value between both electrodes is zero, while the liquid crystal molecules are inclined in an electric-field direction as the effective voltage value is increased, so that the optical rotation disappears. For this reason, for example, in a transmissive liquid crystal panel, polarizers of which polarizing axes are perpendicular to each other are disposed correspondingly to an alignment direction at the incident side and the rear-surface side, respectively (normally-white mode). In this case, when the effective voltage value between both electrodes is zero, the light is transmitted and thus a white color is displayed (the transmittance is increased), while the quantity of light to be transmitted is decreased as the effective voltage value is increased and thus a black color is displayed (the transmittance is minimized). Therefore, by controlling the voltages applied to the pixel electrodes in a unit of pixel, a predetermined display is possible.
However, in the liquid crystal panel, when the thickness of a liquid crystal layer (that is, a cell gap) is not uniform, for example, as shown in FIG. 11A, brightness difference occurs even if the same brightness is intended to be displayed all over the pixels, and the brightness difference is visible as unevenness of brightness. In the light and darkness, here, it is dark when the liquid crystal layer is thin and it is bright when the liquid crystal layer is thick, but when the mode is changed, this relation may be reversed.
In order to make the unevenness of brightness invisible, there has been suggested a technique of making the brightness of pixels uniform by adding correction signals for increasing brightness to image signals supplied to the dark pixels.
There has also been suggested a technique of digitally processing the above correction. In this technique, data indicating a brightness correction amount are stored in advance for each pixel (every area divided plurally) of the liquid crystal panel. When an image signal is supplied to an arbitrary pixel, data corresponding to the pixel are read out, the correction amount thereof is added to the image signal, and then the added signal is supplied to the pixel. Specifically, when the unevenness of brightness shown in FIG. 11A occurs, for example, the correction amounts shown in FIG. 11B are added to the image signals of the pixels belonging to the respective areas. In FIG. 11B, the correction amounts are values obtained by expressing voltage data to be added to the image signals in decimal values.